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AS T

MY AND ION NO O S RO

N E CE TER ER PH

N AT I O N A L
AR EC RY IB O TO O B S E R VA

November 2000, Number 31
mation on key physical processes at work in the comet: the heating and sublimation of ice in the nucleus and the heating of water vapor in the coma. In comets, excitation of the 18-cm OH line involves absorption of solar UV photons and a subsequent radiative cascade to the ground state. In highly productive comets, OH spectral lines in the inner coma appear weaker than expected because gas collisions thermalize and "quench" this maser emission. While it has been observed that such quenching plays a major role in extremely productive comets (such as Comet Hale-Bopp), the goal of the study described here is to achieve high-resolution observations of a less-productive, more "typical", long-period comet, in order to investigate the degree of quenching. Amy Lovell (Agnes Scott College), Pete Schloerb (FCRAO) & Ellen Howell (NAIC) observed Comet C/1999 S4 in early May 2000, and again in late July and early August, around the time of the spectacular breakup of the nucleus. Arecibo is well suited to these observations, yielding good signal-tonoise, as illustrated in the observed spectrum for this "typical" comet (Fig. 1).

Radio Astronomy Highlights Chris Salter Cometary Radio Astronomy Observations of OH lines at 1665 and 1667 MHz can provide much useful information about comets: the total number of OH molecules in the coma, the rate and distribution of sublimating water ice from the comet nucleus, and the velocity of gas in the coma. Taken together, these observations provide infor-

INDEX
Radio Astronomy Highlights ........... 1 Solar System Studies ........................ 9 Space and Atmospheric Sciences .. 11 State of the Observatory ................ 13 Peeps into the Past .......................... 15 Summer Students 2000 .................. 16 Gregorian Focal Plane Array ........ 19 RFI Efforts ...................................... 19 Recent Colloquia ............................ 19 Computer Department News ........ 20 Control Room News ....................... 20 Comings and Goings ...................... 20 Single Dish School Announcement 21
Fig. 1: The 1667-MHz line of OH in Comet LINEAR (S4) on 1 June, 1999, before the nucleus broke apart. These data result from a 105-min total integration on the comet photocenter. A linear continuum was fitted and subtracted, and the resulting spectrum smoothed by 0.3 km s ­1. The OH line shows a velocity dispersion of about 0.9 km s ­1. (Courtesy E. Howell)

The NAIC is operated by Cornell University under a Cooperative Agreement with the National Science Foundation.


Preliminary modeling results reveal that there is some degree of quenching in the coma, as expected, and production rates are consistent with those reported from other sources. HI in the Magellanic Stream Snezana Stanimirovic (NAIC), John Dickey (Univ. of Minnesota), Alyson Brooks (Macalister College), Abby Hedden & Amanda Kirchner (Carleton College) have mapped two regions at the tip of the Magellanic Stream (MS). The MS is a huge tail of neutral hydrogen (HI) trailing behind the Magellanic Clouds, and stretching across almost 100 degrees on the sky. It is generally believed that the MS is the result of the wild interactive past of the Magellanic Clouds with our Galaxy, but consensus still has not been reached on the exact form of these interactions. The present project has two

main scientific objectives. Firstly, to investigate how much the Galactic Halo influences the properties of the gas in the MS. Secondly, as a hierarchy of structures is present in the diffuse interstellar medium of our Galaxy and many other galaxies, these observers wish to probe the structure organization of an almost primordial environment like the MS in which no stars have yet been found. On-the-fly mapping was undertaken of two regions in the MS, known as MS V and VI, driving in RA and stepping in Dec. Each individual map, containing 10â11 pointings, was separately reduced using a combination of ANALYZ and AIPS. The integration time per pointing of 30 sec gave an rms noise of 0.1 K. Ultimately, all the small maps were combined together. The preliminary results are: 1. MS V clump (Dec ~8.5°): This region (Fig. 2) shows a strong ridge of gas almost 80° away from the Stream's head near the Magellanic Clouds. Most absolutely fascinating is that along the whole

region covered a is found of about The two parallel gether in several like features.

bifurcation in velocity 25 km s ­1 (see Fig. 3). sheets of gas meet toplaces via horseshoe-

2. MS VI clump (Dec ~13°): Only 5° further along the MS, its structure looks very different. Instead of sheets of gas, clumps of various sizes, as well as filaments, are seen. The major clump in the region (Fig. 4) shows a velocity gradient in the position-velocity slices reminiscent of rotation curves seen in many galaxies. If its complex velocity field is indeed due to rotation, this would be a real surprise for the Stream theories. In the same region, we have found one of the so-called head-tail clouds which exhibits simultaneously density and velocity gradients. Similar clouds have been seen elsewhere in the MS (Bruns et al. 2000, A&A, 357, 120) and were explained as currently interacting with the ambient medium of the Galactic Halo. 3. Statistical investigation: To investigate the organization of structure in the MS they have determined the spatial power

9°0'

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8°0' 23h29m30s 23h28m30s 23h27m30s

Fig. 2: The HI column density image of the MS V region in the Magellanic Stream. The maximum column density is 9.5â1019 atoms cm-2. The contour levels range from 1.1 to 9.2â1019 atoms cm-2, with a contour interval of 1.6â1019 atoms cm-2.(Courtesy S. Stanimirovic)

Fig. 3: The position-velocity diagram of the MS V region in the Magellanic Stream. Two almost-parallel sheets of gas are seen, separated by about 25 km s-1. (Courtesy S Stanimirovic)

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Clouds (HVCs) and Local Group dwarf galaxies. Recent evidence has suggested that the HI around nearby dwarf spheroidal galaxies might be related to the long-known but poorly-understood phenomenon of HVCs. However, very few examples of either type of object have been observed with a telescope large enough to resolve the HI emission, and thus only a limited amount is known about this gas. Therefore, these observers are using Arecibo to make high-resolution maps of the neutral gas content of all accessible Local Group dwarfs, and a selected sample of compact HVCs. With the upgraded system at Arecibo, they can map large areas (a square degree or more) in reasonable amounts of time (1-2 hr), while retaining the sensitivity necessary to detect faint sources. Their image of HVC 57, selected from the Wakker & van Woerden catalog (1991, A&A, 250, 509) is displayed in Fig. 5. The only previously existing observations of this HVC were made with the Dwingeloo 25-m telescope with a 36-arcmin beam, so that the HI appears

Fig. 4: The HI column density image of the MS VI region in the Magellanic Stream. The maximum column density is 5.8 â 1019 atoms cm-2. The contour levels range from 6.1 to 49.0 â 1018 atoms cm-2, with the contour interval of 8.6 â 1018 atoms cm-2. (Courtesy S Stanimirovic)

spectrum for the MS VI region. Similar statistical analyses have been performed for our Galaxy and several other galaxies, including the Small Magellanic Cloud. The spatial power spectrum of the HI column density distribution in MS VI can be fitted by a power law having a slope of ­3.7. This is significantly steeper than for the Small Magellanic Cloud, whose slope is ­3.3 (Stanimirovic et al. 2000, MNRAS, 315, 791), and also extremely close to the power-law index for Kolmogorov turbulence. Power-law behavior of the spatial power spectrum is usually ascribed to interstellar turbulence. However, what causes this turbulence is still not well understood. HI in High Velocity Clouds (HVCs) Leo Blitz, Josh Simon (Berkeley), Tim Robishaw (Maryland) & Carl Heiles (Berkeley) are in the midst of an observing program focusing on High Velocity
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Dec Offset (arcmin)

RA Offset (arcmin)

Fig. 5: The Arecibo map of the HI column density of the High Velocity Cloud, HVC 57, selected from the Wakker & van Woerden catalog (1991, A&A, 250, 509). The total integration time for the whole map was 164 min. (Courtesy Leo Blitz)

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merely as a single point, with unknown spatial extent or brightness temperature. The Arecibo beam clearly resolves the HVC, revealing it to be much smaller than previously known and actually consisting of three separate clumps. These clumps are connected by very weak bridges, and there is some diffuse emission visible to the north. Each clump has a different velocity, which will allow these observers to investigate the kinematics of the gas in detail. They intend to use the large amount of information that can be extracted from these observations to constrain models for the origin of HVCs. In addition, their dwarf galaxy observations will enable them to study closely the relationship between the stars and the newly-discovered gas in these objects. HI in Star-Forming Galaxies Over the past few years it has been realized that many of the early-type galaxies in nearby clusters have experienced a surprising variety of star formation histories. In particular, a recent study by Concannon, Rose & Caldwell (2000, ApJ, 536, L19) has revealed that there is an apparent segregation by mass in that the smallest, low-mass systems show the widest range in past star formation histories. While the majority of the galaxies in their early-type sample have mean stellar population ages of 7-10 Gyr, nearly 25% of the sample, particularly those of the lowest mass, have experienced an episode of star formation within the last few Gyr. The bursts of star-formation experienced by the low-mass galaxies appear to be linked to the cluster environment, but little is known about the frequency with which the episodes occur or what mechanism is responsible for inducing the starbursts. However, recent Arecibo HI observations are beginning to shed light on these unresolved issues. A key observation that can determine the nature of the star formation and the triggering mechanism is to evaluate the amount of gas present in the sample of galaxies during various stages of their
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VCC571

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1

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-1

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Fig. 6: The HI spectrum of the galaxy VCC571. (Courtesy Kristi Concannon)

star formation history. In fact, Kristi Concannon, Alejandro Gaba and Jim Rose (North Carolina) have begun such an Arecibo program to investigate the HI content of their sample of star-forming and post-starburst, low-luminosity Virgo galaxies. The goal of these observations is to determine whether the older, post-starburst galaxies retain enough gas after the star formation episodes so that the bursts can recur, or whether the galaxies are completely stripped of their gas reservoir during the star-forming event. By measuring the amount of gas present in galaxies with various histories, it is possible to place constraints on processes that may be occurring in the cluster environment and inducing the episodes of star formation. In June 2000, Concannon, Gaba and Rose obtained HI spectra of 20 of their sample galaxies at various stages in their evolutionary process. Of these, one definite detection (at the 6 level) and six somewhat ambiguous detections (at the 1.5-2.5 level) were made (Fig. 6). The HI mass detected in these galaxies is between 8â106 and 5â107 M , which for galaxies with ­17.7 < MB < ­16.7, cor-

responds to a HI gas fraction (MHI/LB) within the range 0.01 and 0.08. If the detections are real, then what they find is that the majority (80%) of the detections are galaxies with stellar populations that are of intermediate to young ages (1-7 Gyr). In fact, more than 50% of the galaxies in this age range exhibit detectable HI masses. This seems to suggest that for a large fraction of the galaxies, their initial gas supply is exhausted during a star formation episode and is not replenished later in their lifetime, otherwise the new supply of gas would be observed as it accumulates in the galaxies with older stellar populations. From this information, Concannon et al. conclude that mechanisms such as minor mergers or galaxy harassment, which result in the funneling of gas to the nucleus of the galaxy and a centralized starburst, may be driving the episodes of star formation. The observations for the remaining galaxies in their sample will be made in the coming months. A study of the HI gas content of a volume-limited sample of actively starforming dwarf galaxies is being carried out by Janice Lee (Arizona), John SalzNAIC/AO N ewsletter

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Fig. 7: HI mass versus redshift for a sample of actively star-forming galaxies. (Courtesy Janice Lee)

er (Wesleyan), Chris Impey (Arizona), Trinh Thuan (Virginia) & Caryl Gronwall (Johns Hopkins). The sample is drawn from the first list of the KPNO International Spectroscopic Survey (KISS) (Salzer et al. 2000, AJ, 120, 80, and AJ, submitted), a deep objectiveprism survey for emission-line galaxies. Due to its high sensitivity, KISS detects dwarf galaxies which have strong H- emission lines to velocities well in excess of 10,000 km s ­1. For this study, all star-forming galaxies with velocities less than 11,000 km s­1, and blue absolute magnitudes fainter than ­18.0 (N = 108) have been targeted for 21-cm observations. While previous HI analyses of star-bursting dwarf galaxies have utilized samples comprised of objects from a number of different catalogs, the current study will examine a set of objects which not only have been cataloged in a uniform manner, but also have a well understood selection function. This will help ensure that the results are represen-

tative of the entire population of this class of galaxy. Among the questions this team is trying to address are the following. What are the connections, if any, between the bursting dwarf galaxies and more quiescent dwarf irregulars and dwarf ellipticals? If the starbursts seen in these dwarfs repeat, then what is the fraction of time that they are active? Can this deep sample of dwarf galaxies be used to constrain the low-mass end of the HI mass function? To date the observations for this project are roughly 60% complete. Because of the low HI masses involved, and the more distant nature of the sources, this sample pushes the sensitivity limits of the upgraded Arecibo telescope to the extreme. As seen in Fig. 7, many dwarfs with HI masses below ~108 M have been detected at distances well beyond the Local Supercluster. Once the observations are completed, this sample will represent one of the deepest HI surveys for dwarf galaxies ever undertaken.

HI in Low Surface Brightness (LSB) Galaxies One of the most intriguing classes of LSB galaxies is the supergiant disk systems, the Malin class. The prototype to this class, F568-6, was discovered from a visual search of the PSS-II (Bothun et al., 1990, ApJ, 360, 427). While low in central surface brightness (µo = 23.4 B mag arcsec-2), F568-6 is by no means low in luminosity (MB = ­21.1), nor low in total or HI mass. The Malin class contains the largest galaxies in the Universe, yet are notoriously difficult to find and catalog (Sprayberry, Impey & Irwin, 1996, ApJ, 463, 535). One of their properties provides a promising avenue for the cataloging of a significant number. Most Malin-class galaxies have a weak AGN in their core. The current theory is that the copious gas supply in the disk provides the fuel for a central engine, even if at a low intensity (Schombert, 1998, AJ, 116, 1650). While the weak AGN appears as a point source, its nearIR colors would distinguish it from a stellar SED. Unfortunately, a survey of the sky in the near-IR will not, by itself, identify the Malin objects since the sky brightness at 2.2 µm is 2000 times higher than in the optical making their disk regions invisible. Thus, this project requires a "virtual observatory", in this case the combination of two existing databases, DPOSS (optical) and 2MASS (near-IR). The procedure is straight-forward, first isolate all the objects in the 2MASS catalog with non-thermal colors, (i.e. outside some boundary defined by normal stars). Second, search the near-IR source positions on the blue plates of DPOSS with a fast area scan. A series of circular apertures are placed around t he point source t hen tested against the local sky. LSB galaxy detection is effective if only a particular region is being tested against the background since varying diameters are checked which maximize the signal from the LSB disk versus the signal from sky. A preliminary search was undertaken last winter using eight plates from DPOSS that contained some fraction of 2MASS coverage (about 3 deg2). Forty
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candidates were produced of which ten were searched with the new, upgraded Arecibo telescope by Jim Schombert (Oregon), JoAnn Eder & Karen O'Neil (NAIC). The Gregorian system at Arecibo has a wide velocity range, a critical element since the large Malin objects tend to be at velocities greater than 8,000 km s ­1. Eight of the ten candidates were detected at 21-cm. Four of the DPOSS images are shown in Fig. 8 and a typical detection in Fig. 9. All have the characteristic AGN nucleus surrounded by a LSB disk. All eight also have HI widths in excess of 350 km s-1 (the typical spiral has a rotation width of 250 km s ­1). Assuming that these galaxies follow the baryonic TF relation (McGaugh et al., 2000, ApJ, 533, 99), then their masses will exceed 1012 M . In similar vein, over the past year Karen O'Neil (NAIC) & Greg Bothun (Oregon) have been using the Arecibo Gregorian system to look for low surface brightness galaxies whose morphologies indicate that they should have a reasonably high gas content, yet whose redshifts are unknown, and which had not been found in previous HI searches at Arecibo or with other telescopes. This project is being undertaken now to take advantage of the higher sensitivity and redshift range of the Arecibo telescope and the L-band narrow receiver. To date 34 galaxies have been searched for, and 16 discovered. The majority of the "discovered" galaxies lie between 12,000 22,000 km s ­1 away, making many of these systems considerably more massive than previously believed. Preliminary results of this survey, then, have doubled the number of known Malin-1 type galaxies in the Universe and indicate that low surface brightness galaxies may contribute considerably to the baryonic mass of the z<0.1 universe. In May, 2000, Kristin Kearns (U. Wisconsin, Superior) & Karen O'Neil (NAIC) used 40 hr with the L-wide receiver to search for HI emission in the directions of approximately 200 previously uncataloged, moderately low surface-brightness galaxies (< 24-24.5 B

Fig. 8: The DPOSS images of four HI detection from the search for Malin objects by Schombert, Eder & O'Neil. (Courtesy Jim Schombert)

Fig. 9: A typical detection from the search for Malin objects by Schombert, Eder & O'Neil. The optical counterpart of this galaxy, J651_13_12_270_761, is shown in the top left-hand frame of Fig. 8. (Courtesy Jim Schombert)

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mag arcsec-2) selected from the optical images of the Kitt Peak International Spectroscopic Survey for emission line galaxies (KISS: Kniazev et al. 1998, IAU Symposium 179, 302). In total, more than 1000 candidate objects have been identified within the 102 square degree KISS strip. This run was used to observe a set of candidates representative of the full list, in order to identify the candidate characteristics which yield the highest HI detection rate, in order to prioritize the target list. As a result, the total detection rate for this run was low (~13%). It became apparent, though, in comparing the optical appearances of the detections with those not detected, that the statistics could be improved considerably (up to 50%) by eliminating from the target list those galaxies with the optical morphology of background objects, through a minimum size requirement (rminor > 10 arcsec). Additionally, as a number of objects were found near the high-velocity limit of 18,000 km s ­1, observing a wider bandwidth (1260-1430 MHz) and doubling the observing time per object from 5 to 10 min should bring the detection rate in the future close to 60%. The extended frequency range now available on the L-narrow receiver will also help. Along with the rest of our avid Newsletter readers, these observers are hoping to have time scheduled again this season to continue this most exciting project. Molecular-Line Studies On September 15-18, 2000 Peter Hofner, Esteban Araya (UPR -- Rio Piedras), Ed Churchwell (Wisconsin) & Stan Kurtz (UNAM) used the Arecibo C-band system to observe simultaneously the ground-state transition of H2CO and the hydrogen recombination line H110 at rest frequencies of 4829 and 4878 MHz respectively toward a sample of 21 ultracompact HII Regions. During the run they frequently observed calibration continuum sources with known flux densities to verify the performance of the C-band system. Based on these observations, the gain of the telescope varied between 2.0-2.5 K/Jy during the obserNovember 2000, Number 31

Fig. 10: A sample uncalibrated spectrum of the H2CO 1(10)-1(11) line toward the source G37.87-0.40. (Courtesy Peter Hofner)

vations. Both H2CO absorption and H110 were easily detected towards 20 of the 21 sources observed. These data will allow derivation of improved distance estimates toward this target sample of ultracompact HII regions. Another goal of this project is the comparison of the extended ionized gas, which is sampled here in continuum and recombination line with a beam of about 1 arcmin, with the properties of the ultracompact HII regions which were defined entirely on previous interferometric observations that are insensitive to any extended structure. Fig. 10 shows a sample uncalibrated spectrum of the H2CO 1(10)-1(11) line toward the source G37.87-0.40. The data are presently being reduced and analyzed. The Arecibo OH megamaser survey being made by Jeremy Darling & Riccardo Giovanelli (Cornell) is now approaching completion, with less than 40 candidate OH megamasers (OHMs) as yet unobserved. They have detected 42 new OHMs in the survey to date, and at least 3 cases of time variability. Included in the new detections are a few OH

"gigamasers", one of which is the most luminous OHM ever detected. In a separate recent project, Jeremy & Riccardo have attempted to detect OHMs in nearby AGN which are undetected by IRAS. These measurements reveal no OH lines associated with AGN in quiescent (non-interacting) systems. The observations place stringent limits on the influence of AGN in forming OHMs in non-interacting galaxies and give us some confidence in the technique of using OHMs as tracers of merging systems. Steep Spectrum Continuum Sources David Kaplan (CalTech), Jim Cordes (Cornell), Jim Condon (NRAO), Chris Salter & Mike Davis (NAIC) have used Arecibo to measure the spectra and variability of radio sources from a complete sample with extremely steep spectra. This sample is interesting because of its statistical properties, and due to the steepness of the spectra. The sources push the 1012 K limit, and require accurate measurements of the spectral indices and the locations of the flux-density
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maxima to settle this issue. Also, these sources could be variable on month-year time scales, and this obviously affects the completeness. The team observed the sources with all of the Arecibo Gregorian receivers from 430 MHz to 5 GHz, using several bands in the wide receivers (e.g. L-wide and C-Band). The degree of automation achieved allowed very smooth observations, with 40 sources being observed over 2 days. The observations were then repeated 2 months later to examine variability. While the data are not yet completely analyzed, initial results are promising. Continuum fluxes have been measured down to few mJy at 5 GHz, and the recently improved 610-MHz focus was immediately noticeable. These observations seem very interesting, and will hopefully pave the way for Arecibo to regularly make rapid, multi-frequency continuum measurements. Pulsar Research Measured masses of neutron stars fall in a remarkably small range, 1.35±0.05 M . The best such measurements come from neutron star--neutron star binaries, such as the Hulse-Taylor system. By contrast, the few measurements of neutron star masses in neutron star--white dwarf binaries tend to have lower precision. Masses of neutron stars in these binaries are of particular interest because these systems go through an extended period of mass transfer, during which the secondary loses several tenths of a solar mass of matter, at least some of which is accreted onto the neutron star (making it visible as a low mass X-ray binary). Thus, one might expect these neutron stars to have masses substantially above 1.35 M . In a paper recently submitted to the Astrophysical Journal, David Nice, Eric Splaver (Princeton) & Ingrid Stairs (NRAO) report on PSR J2019+2425, a millisecond pulsar in a 76-day orbit with a white dwarf. Pre-upgrade observations used the Princeton Mark III pulsar timing system. Post-upgrade observations, part of the coordinated pulsar timing
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project, used the Princeton Mark IV system. Observations were made on 78 days over a nine-year period. Times of arrival were calculated from the data and were fit to a standard model of pulsar rotation, astrometry, and orbital motion. Among the observed orbital parameters of the system is the projected semimajor axis, x=(a1 sin i)/c, where a1 is the semi-major axis of the pulsar orbit, i is the inclination of the orbit relative to the line of sight, and c is the speed of light. Pulses arrive earlier or later by x sec over the course of the orbit. The data show a small but significant change in x over · /x=1.3±0.2â10­15 s-1 time, quantified as x¤ (see Fig. 11). This is due to the relative proper motion of the system and the earth. Combining this measurement with the known proper motion of the system, µ=22.6 mas yr-1, constrains the inclination angle to be less than 72 degrees. Broadly speaking, this is because a highinclination, edge-on orbit could not show a change in the projected semi-major axis as large as observed. A highly edgeon orbit would also produce a detectable Shapiro delay signal, which is not seen. Models of the accretion phase of the binary system predict a relation between the orbital period and the white dwarf mass, dependent on the composition of the dwarf. For the PSR J2019+2425 system, the predicted mass lies in the range 0.31 to 0.35 M , with the higher figure for a population II star. The 0.35 M

upper limit on the white dwarf mass and observed constraint i<72°, imply the neutron star mass is no more than 1.51 M . The median likelihood mass is only 1.33 M , according to a Monte Carlo calculation which considered all allowed orientations of the binary system. This finding adds to a growing body of evidence that pulsars in neutron star--white dwarf binaries are not much more massive than those in neutron star--neutron star binaries. Andrea Lommen & Don Backer (Berkeley) recently observed 4 strong millisecond pulsars (PSRs J1022+1001, J1713+0747, B1855+09.and B1937+21) using the S-band wide and C-band receivers. High frequency observations are less affected by variations in interstellar propagation (dispersion and scatter) and so will inherently provide better timing, with the caveat that the signal-to-noise ratio (S/N) be maintained at higher frequency. This is non-trivial given that most pulsars have a steep spectral index, and therefore a significant drop-off in flux with increasing frequency. While impractical at low frequency due to the short time scales, at high frequency it is possible to improve S/N by chasing scintillation maxima, or scintles. Scintle hunting involves looking over a large bandwidth (~1 GHz), finding the brightest scintle, and taking timing or other data centered at that frequency. Scintillation bandwidths (Bs) scale with frequency to the fourth power while

Fig. 11: Residual pulse arrival times of PSR~J2019+2425 after removing a model with fixed orbital size. The
· =5 solid line shows the r sidual arrival times corresponding to the observed change rate of orbital size, x¤ â

10-14. The curve is sinusoidal with the orbital period, with amplitude increasing linearly towards the ends of the data set. (Courtesy David Nice)

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PSR J1713+0747 5700 2610

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Fig. 12: (Left) PSR J1713+0747 across nearly 2 GHz of bandwidth using the C-band receiver. The vertical plot shows the peak S/N for each of the 288 individual profiles which were acquired in 9 steps of the 112-MHz ABPP. (Center) PSR B1855+09 across nearly 1 GHz of bandwidth using the S-wide receiver. Peak S/N is shown for each of the 192 individual profiles, which were acquired in 6 ABPP steps. There is a factor of as much as 30 in S/N to be gained by roving in frequency. The gaps in frequency are regions of the bandpass that were skipped in order to avoid interference. (Right) PSR B1937+21 across more than 1 GHz of bandwidth using the C-band receiver. Peak S/N is shown for each of the 192 profiles from 6 ABPP steps. A scintillation bandwidth of approximately 100 MHz is evident.(Courtesy A Lommen)

scintillation time scale (Ts) are proportional to frequency. In principle, at high frequencies there should be objects which have a long enough Ts and wide enough Bs that scintle hunting is practical. Preliminary results of the observations for three of the pulsars are shown in Fig. 12. The left panel of this figure shows a scan of the 2-GHz bandwidth available at 5 GHz for PSR J1713+0747. The Arecibo-Berkeley Pulsar Processor (ABPP) observes 112 MHz of bandwidth at time, in 32â3.5-MHz channels. Each horizontal line represents a single 3.5 MHz coherently dedispersed channel of the ABPP, and each batch of 32 channels was taken in succession. The pulsar appears not to scintillate at 5 GHz. The variation seen across this plot is monotonic with frequency and is the combined effect of the spectral index of the pulsar and the frequency dependent gain of the receiver. Slight, but regular, dips in S/N are due to the fall off in the passband of the ABPP. For the other two pulsars shown in Fig. 12, scintillation effects are most
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prominent. For B1855+09 (center panel) there are variations in S/N by as much as a factor of 30 at S-band. This theoretically translates directly to an decrease in RMS of timing residuals by the same factor. Bs is on the order of 100 MHz and Ts is longer than the 1-hr observation time. Note that the S-wide receiver bandpass does contain regions of RFI, but these are stable in time, and it was possible to consistently avoid them. For B1937+21 (right-hand panel) it is seen that the scintillation bandwidth is about 100 MHz at 5 GHz, so if one can locate a bright scintle and observe there, one could in principle improve S/N by a factor of 3 over the average. However it was found that the scintillation timescale for B1937+21 at 5 GHz is less than 15 minutes. For J1022+1001 at S and C-band, and J1713+0747 at L-band. Scintillation is observed, but there is only a factor of 2 or 3 in S/N to be gained by observing bright scintles. Lommen and Backer have a preliminary scintle hunting mode that uses the Berkeley Arecibo Caltech Swift Pulsar

Instrument (BACSPIN) in conjunction with ABPP. BACSPIN's 168-MHz capability to quickly scan the available bandwidth (1 GHz at S-band, 2 GHz at C-band) is used to produce a `snap-shot' from which to choose the best frequency for ABPP. Both ABPP and BACSPIN are public-access machines and Lommen and Backer are happy to help people continue to develop and use this capability. Contact either of them for further details (andrea@astron.berkeley.edu; dbacker@astron.berkeley.edu). Solar System Studies Don Campbell The October meeting in Pasadena of the Division of Planetary Sciences of the American Astronomical Society was the first to highlight many of the successes of Arecibo's upgraded planetary radar system. Papers were presented by Chris Magri (U. Maine, all the papers had multiple authors) on constraints on the composition of 45 mostly main belt asteroids from radar observations, and by Mike Nolan (NAIC) on recent Arecibo delay-Doppler imaging of main belt ob-

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jects. Jean-Luc Margot (NAIC) discussed recent Arecibo near-earth asteroid observations including the orbital properties of the first radar detected binary asteroid (see below), and Steve Ostro (JPL) presented a poster o n Arecibo and Goldstone imaging observations of the near earth asteroid 4486 Mithra. Mithra appears to be a highly bifurcated object possibly in a very unusual spin state. Moving much further out in the solar system, Phil Nicholson (Cornell) showed the first radar delayDoppler images of Saturn's rings, which were obtained with Arecibo last fall, and Don Campbell (Cornell/NAIC) discussed the first 13-cm wavelength radar detection of the surface of Titan which was also obtained last fall. The Saturn's rings images clearly show azimuthal asymmetry which is thought to reflect subtle dynamical effects in the rings related to particle clumping and their subsequent destruction and shearing due to the gravitational gradient across the clump. The Titan results indicate that the satellite's surface has radar scattering properties that are intermediate between those of the icy Galilean satellites of Jupiter and the terrestrial planets and are a considerable puzzle. There is clear evidence that the radar reflectivity does change over the large surface feature centered at about 120 degrees of longitude that was discovered a few years ago in optical observations from the earth and the Hubble Space Telescope. New Arecibo radar observations of Saturn's rings and Titan will take place in November of this year. The major excitement of the past month was the discovery of the first radar detected binary near earth asteroid. The asteroid, 2000 DP 107, was discovered in February of this year by the Lincoln Laboratories/MIT near-earth object search program, LINEAR. On the basis of optical observations its size was estimated to be approximately 1 km and it was predicted to make a close approach to the earth on September 19 at a distance of 0.05 AU (about 7.5 million kilometers or 20 times the distance to the

moon). Unfortunately, close approach was at southern declinations but DP 107 moved to northern declinations and, hence, into Arecibo's view a few days after close approach when it was at a distance of about 0.09 AU from the earth. While there was nothing especially unique about DP 107 from the early optical observations, Jean-Luc Margot and Mike Nolan of NAIC's staff plus Steve Ostro, Lance Benner, Ray Jurgens, Marty Slade and Jon Giorgini, all of JPL, initiated a request for time on the 3.5cm wavelength NASA/JPL Goldstone solar system radar for initial observations, to be followed up by Arecibo observations about one week later if DP 107 proved interesting. The Goldstone observations, which were on September 22 and 23, showed two clearly separated scattering centers which appeared to move relative to each other, strongly suggesting that DP 107 is a binary asteroid. Follow-up observations with the Arecibo 13 cm wavelength radar start-

ed on the evening of September 30 and continued for the nine subsequent evenings. These observations clearly showed that DP 107 is a binary asteroid and a preliminary analysis of all the radar data indicate two bodies of about 800 m and 350 m in diameter orbiting each other with a period of approximately 1.7 days at a separation of roughly 2.7 km (see Fig. 13). Optical observations, primarily by Pravec et al. using the 0.65-m telescope of the Ondrejov Observatory near Prague, provided confirmation of the binary nature of DP 107 from observations of eclipse events which result in a detectable reduction in the reflected light from the system. Based on the existence of paired impact craters on the earth and other bodies, it has been proposed for some time that up to 10% of asteroids may be in binary systems. That binary asteroids exist is somewhat of a puzzle and they are of considerable interest due to the constraints that they may place on asteroid collisional processes. Prior to the discovery that DP 107 is a binary only two other binaries were known with the Ida/ Dactyl system discovered by the Galileo spacecraft being the most well known. The other, 45 Eugenia is a main-belt object whose binary nature was discovered using an adaptive optics system on the Canadian-French-Hawaii telescope on Mauna Kea in 1998. Two other optically discovered main-belt binaries were announced simultaneously with the DP 107 announcement. DP 107 is the first known near-earth asteroid binary although there have been suggestions of others based on possible occultation or eclipse events seen during optical observations, primarily by Petr Pravec and his co-workers in Prague.

Fig. 13: Arecibo range-Doppler image of the binary asteroid 2000 DP107, showing the front cap of a roughly spherical primary and the secondary orbiting at a distance of a few kilometers. Range from the observer increases down and Doppler increases to the right. The rotation of each object spreads the echo in Doppler frequency and gives an exaggerated ratio of sizes in that dimension due to spin periods of less than three hours and more than a day for the primary and secondary, respectively. The actual ratio of sizes, measured from the range extents, is about 3 to 1. (Courtesy Jean-Luc Margot)

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Space and Atmospheric Sciences Don Farley 430-MHz Radar Dual Beam A much anticipated new capability at the Observatory splits the radar beam into two, using both the existing line feed and the new Gregorian feed. The hardest part of this job, dividing the transmitter power and delivering it to the two feeds, has now been done and the Gregorian feed passed its first test in late October. We had hoped to have the dual beam fully operational by now, of course, but life is never quite as easy as one hopes it will be. Successfully transmitting is a major hurdle passed. It is no small matter to extract 1 MW or more, with little reflected power, from a curved, slotted waveguide using a device, designed and built in-house, that moves along inside the guide as the Gregorian moves, and then get the power down into the turret structure inside the radome. We still need to install a 430 MHz radar receiver in the Gregorian along with a transmit/ receive (T/R) switch, and develop the software needed to handle the two simultaneous data streams. We don't anticipate any serious problems with either of these two tasks and so hope to be ready to do real observations by early in 2001. The data acquisition question is discussed further in a separate section below. Having two beams will make a huge difference in the quality of plasma drift velocity data, and thereby virtually all studies involving electric fields and winds, as well as any study in which horizontal gradients are important. Perhaps surprisingly, the data quality in each beam will be nearly as good as it is now (except for high altitude measurements), because the signal-to-noise ratio will still be high after the transmitted power is split. The big difference will be that different azimuth angles will be sampled twice as often as they are now in any beam swinging measurement. Since the mechanical motion of the azimuth arm is slow, when only a single beam is used

the ionosphere itself can sometimes change substantially between samples at a given azimuth. When that happens the assumptions underlying the conversion of line-of-sight velocities to velocity vectors are no longer valid. Simulations have shown that the improvements in the velocity data that the dual beam will provide should often be dramatic, much more so than one might expect. More details can be found at the NAIC web site -- follow the links to: Scientific Users SAS Incoherent Scatter, then scroll down to Data Analysis. Future studies of sporadic-E layers and related plasma instabilities, as well as the unusual F-region structure that sometimes appears over Arecibo, will be greatly aided by having two beams operating simultaneously. Ionospheric Interactions As most readers of this newsletter know, the old RF heating facility in Islote, near Arecibo, no longer exists. Studies with a view to restoring an ionospheric interactions capability, using the main Arecibo reflector, have been underway for some time now. Initial antenna studies indicate that it would be possible to generate a satisfactory beam pattern at 3, 5, and 8 MHz using 2-element Yagis (driven dipoles plus reflector) to feed the main spherical reflector antenna. The next stage of planning, which is now nearly complete, involves more detailed studies, both electrical and mechanical, designed to assure us that it really is feasible to build a feed that can be raised and lowered repeatedly during a campaign in a reasonable amount of time, and that can be stored easily between campaigns. The goal is to build something that will disrupt the astronomy program, and even daytime SAS measurements, as little as possible. The heating experiments take place almost exclusively at night, and often for only part of the night, and it is critical to utilize the remaining hours as much as possible.

In the interests of moving forward quickly, we have decided to design for transmission at only 5 and 8 MHz, for the time being at least. We may consider adding the huge structure needed for 3 MHz at a later date. At the time of writing it appears that the feasibility question for the 5- and 8-MHz feeds will be answered positively. The next step is to get accurate cost estimates so that we can submit a proposal to the NSF. We hope to be able to generate such a proposal sometime in the next few months. 430-MHz Klystron Status It is nice to be able to report that the klystron rebuilt by CTL in California at a cost of about $60K is working pretty well. We had some misgivings about its performance at first, but it seems to have cleaned up with the passage of time and is generating about 1 MW quite reliably. It is a relief to have two operating tubes again, but we still have no spare. It looks like we will have access to a new supply of used, but useable, military klystrons sometime in the spring of 2001, which is good news for Arecibo and for the Millstone Hill Observatory. Data Acquisition System Some changes are necessary in the aeronomy data-taking system so that we can process the samples from two feeds at once. We are using this opportunity to make the first of two planned major changes in the data-taking system. This first step takes advantage of the huge computing capability available in general purpose desktop computers. We will continue to use our current VME crate to collect samples, but processing will no longer be performed there. Raw samples will go (at up to 5 MBytes/sec) to a computer with several hundred GBytes of disk space and two processors running at 850 MHz. We plan to do the computing there, but since this computer is on the local area network we will be able to use other computers as well when necessary. Furthermore, users who bring their own computers will be able to ac-

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cess the raw and processed data. This first stage should be ready for use by the time the dual-beam system is operational, or maybe even earlier. The second step is to replace our system of analog filters and baseband sampling with IF sampling and digital filtering. This will involve replacing the current VME crate with a new device. We will not do this until the first stage described in the previous paragraph is complete. The two steps are independent; in principle the data processing software need not even know that the hardware has changed. In practice we will implement new analysis techniques which will require new software. ISR Science Notes The last newsletter reported on some intriguing ISR electron line measurements made by Mike Sulzer (NAIC), Sixto GonzÀlez (NAIC), and Richard Behnke (NSF) in July of this year. These are observations that only Arecibo can make. The spectra are generated by sampling fast enough to cover the full bandwidth of both the ion line and plasma line and all frequencies in between--several MHz in other words. Some of these full spectra (not those shown in the last

newsletter, though) are hard to understand based on standard ISR theory, at least the theory for a plasma in complete thermal equilibrium. My own guess (and that's all it is at this point) is that the spectra represent non-Maxwellian electron velocity distributions. If so, the interesting geophysical question becomes why should such distributions exist at altitudes of 1500-2000 km, say, during the night in the summer at pretty low latitudes? What causes them? Motivated by the apparent large discrepancies between MSIS and Areciboderived OI densities reported by Burnside et al. [JGR,96,1991] and more recently by others, Nestor Aponte (NAIC) and GonzÀlez [paper to be submitted, JGR, 2000] recomputed neutral O densities using Arecibo ISR data and the ion energy balance for the moderatehigh solar flux part of solar cycle 22. Surprisingly, this study found no significant long term departure from MSIS of the radar results (see Fig. 14) when the least squares fitting analysis concentrates on the F region peak and the lower topside up to where light ions matter. The goal of these new experiments is to obtain data more suitable for use in extending the energy balance technique into the

lower topside in order to include H+ in the energy balance equation. Optical News Airglow observations that were made during the current period consisted primarily of thermospheric neutral wind and temperature measurements. First was the six-day Lower Thermosphere Coupling Study (LTCS) World Day experiment in September. Then, in October, similar measurements were carried out during the Global Ionosphere-Thermosphere Coupling + Wide Latitude Substorm (WLS) + Space Physics and Aeronomy Research Collaboratory (SPARC) World Day. This was followed up by Energy Balance and Topside Ionosphere observations which added H and OI 844.6 nm photometric measurements. Bob Kerr (Scientific Solutions, Inc.) made a first attempt at neutral He temperature and wind measurements with his Arecibo-based interferometer during these latter observations. LTCS World Day: Some 80 hours of optical observations were accumulated during the nights of the LTCS run, including a couple of additional nights before and after the official world days. We measured both upper and lower thermospheric conditions through FabryPerot observations of atomic oxygen 630.0 nm and 557.7 nm lines, respectively. Additional upper mesospheric temperatures were estimated from spectroscopic observations of O2 and OH band airglow that originate near the base of the thermosphere. Overall, we had reasonably good overlap between the radar and optical measurements during this LTCS study, but September is a notoriously bad month for weather, and we had our fair share of rain and cloud cover. So even though we will have some good radar/optical comparisons, this particular study will be less than optimum. Topside Ionosphere Optical Studies: An experiment to sample the composition, dynamics, and coupling of the topside ionosphere, upper thermosphere, and exosphere at solar maximum was

3

x 10

8

[O] (cm3 )

2.5 2 1.5 1 0.5 6 8 8 x 10 3 2.5 1.09 â [O] 2 1.5 1 0.5 6 8 10 12 Local Time 14 16 18 10 12 14 16 18

Fig. 14: Upper panel: Average neutral densities over Arecibo: 1988-1994. Lower panel: same as the upper panel, but with MSIS densities multiplied by 1.09. (Courtesy NÈstor Aponte)

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completed on October 30 by Bob Kerr, RaÇl GarcÌa and Eva Robles (both NAIC). The experiment makes full use of nested optical instrumentation in the Optical Laboratory along with the 430 MHz transmitter and the topside data taking program. Spectral line profiles of the OI 630.0 nm, H 656.3 nm, and the metastable He 1083.0 nm emissions were executed simultaneously using three Fabry-Perot interferometers (FPI), including the guest IR FPI of Scientific Solutions Inc. Two photometers sampled the H and OI 844.6 nm volume emission rates, and the spectrometer sampled brightness of several NIR spectral features arising from O2, OH, and OI. Zonal wind measurements near the 500 km exobase, using the Doppler shift of the He 1083.0 nm emission, were attempted for the first time. The 1083.0 nm He Doppler profile measurements intend to quantify windenhanced planetary escape, and may be compared to dynamics near 300 km measured by sampling the Doppler shift of the OI 630.0 nm emission. Globalscale enhancement of H, due to the identified enhancement of tropospheric methane (in the past fifty years), is being evaluated using the H brightness measurements. Models projecting twofold enhancement in exospheric H content may be validated using the nearly 20-year database of H measurements at Arecibo, and exospheric H abundance may prove to be a truly global bellwether of global atmospheric change. The H, H+, and O+ abundance and thermal measurements are used to evaluate the
Ruben Delgado

planetary escape flux due to charge exchange ­ evaluating an hypothesis that states that the charge exchange escape flux from earth is particularly significant at low latitude and after the autumnal equinox, where and when H+ composition is relatively high. The He+ data are used with the metastable He line profile data to evaluate the contribution of He+ charge exchange to the metastable helium source, relative to the photoelectron impact source. Similarly, the OI 844.6 nm brightness measurements are used to evaluate the photoelectron flux and to determine O density above the F2 peak in a model dependent method constrained by the ionospheric conditions sampled by the radar. Weather conditions reduced ISR and optical measurements to about 50% of the possible temporal overlap. Difficulties with a balky pointing system further limited the coverage of the 630.0 nm FPI, and optical plate coating imprecision reduced the spectral resolution of the NIR FPI. Nevertheless, a significant amount of simultaneous observations from the nested instrumentation was realized during the two-day experiment. Other Optical News: During the July dark moon period, we also conducted a series of thermospheric wind measurements that will be used to supplement the CEDAR Database at NCAR. Such measurements continue, more or less automatically, whenever the weather permits. New arrival Shikha Raizada (see the previous issue of the NAIC Newsletter) cut her teeth in August in lidar remote sensing with her initial observations of sodium in the upper atmosphere using our Nd:YAG-Dye laser system. Although only a few nights of observations were made, Shikha quickly learned the importance of getting our aircraft safety radar back into operation; staying up late, outside in the dark, continually watching for airplanes, is certainly not the most rewarding part of lidar observations! We also welcomed to our group a new graduate student. RubÈn Delgado

(see photo) is a Master 's student in analytical chemistry at the University of Puerto Rico, Rio Piedras. His chemistry background has already paid dividends; he is delving deeply into the mesospheric chemistry work in the literature. He also has considerable laser and optics experience. He quickly learned the technical parts of the potassium resonance lidar system and has implemented some small but significant improvements in both the transmitter and receiver. With RubÈn, two important issues for mesopause temperature measurements have been addressed. First, we need to optimize the signal strength by making the receiver as efficient as possible. RubÈn, with Jonathan Friedman (NAIC) and Shikha, worked on optimizing the collimation and focus of the potassium lidar telescope. Subtle details learned during this work will soon be applied to the other two pointable telescopes on the lidar lab roof. Secondly, RubÈn has begun to tackle the tricky problem of laser stabilization. He has already improved the locking of the alexandrite laser to the potassium resonance and hopes to make further improvements in the near future. Since safety issues are a major concern for the lidar, we have made two important steps in securing the laboratory activities, both for our workers as well as for over-flying aircraft. We received safety curtains to section off parts of the Lidar Lab to contain the laser beams within certain areas. We also repaired and reinstalled the aircraft safety radar (mentioned above) to make it easier to operate our lidars routinely and lessen the burden on outside spotters. State of the Observatory Paul Goldsmith and Daniel Altschuler Time on the 305-m telescope is dominated by scientific observations, even though commissioning continues to be a significant factor as we work on calibration issues as well as bringing new systems on line. The time breakdown

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for the period July 1, 1999 through June 30, 2000 is as follows Program Radio Astronomy Radar Astronomy Atmospheric Science Commissioning Scheduled Maintenance Unscheduled Maintenance Hours 4954 420 744 922 1531 214

The large amount of time for scheduled maintenance activities reflect the multitude of ongoing projects including installation of 430-MHz waveguide to the Gregorian and work on rotary floor system in the Gregorian. Note that some of the biggest projects of this sort do not directly impact scheduled telescope time. In this category are the repairs to the antenna reflector support cables and panels, closing the central aperture of the antenna, and the photogrammetry of the surface being carried out as part of the program to improve the reflector surface accuracy. The number of hours lost due to unscheduled maintenance is a tribute to our dedicated staff whose equipment generally works well, and who generally are ready to come in at all hours to fix things quickly when there are problems. This is really an impressively low number, especially when you consider the number and variety of systems that we have on the 305-m telescope. Since our first deadline post-upgrade on July 1,1997 until October 1, 2000, we have received 490 proposals. Of these we have scheduled a total of 241, involving 278 different participants from 116 different institutions. In the period July 1, 1999 through June 30, 2000 we supported a total of 194 investigators from 69 institutions. Thus we can see that things are definitely becoming more and more active with each day that passes! The surface accuracy of the reflector has limited observations to frequencies primarily below 2.4 GHz, although there has been considerable interest in the higher frequencies. We are confident that the adjustment of the surface will

considerably improve things during the year ahead. We are thus completing a receiver covering 3 to 4 GHz. This was designed by Gene Lauria (NRAO) before he left NAIC, and Gene is expected back in Ithaca imminently to work with Lisa Wray (NAIC) on final assembly and testing of this important system that will cover lines of the CH radical, among many interesting molecular species. Along with the 4 to 6 GHz receiver already in place, we should then have essentially continuous frequency coverage from 1 through 6 GHz. This last month has also seen installation of a 327-MHz system with 80-K amplifiers. This system is currently being checked out and calibrated. It is expected to see use for VLBI and pulsar observations. Work has started on design of L-band focal plane array receiver. This is discussed in an article on page 19 of this newsletter. We have been frustrated by the extensive delays in the delivery of our new VLBI (VLBA4) system, but at the time of writing things are gearing up for acceptance tests at Signatron Technology. Haystack Observatory has generously agreed to support some testing of the unit there that will ensure that it can write tapes that can be properly correlated. If there are no unexpected further delays, the unit could be shipped to Arecibo by the end of November. Photogrammetry efforts to measure the figure of the main reflector leading to eventual adjustment of the surface for improved perfor mance at high frequencies, have been frustrated by the wet weather. Nevertheless, progress has been made in refining both the techniques and procedures to

analyze the data once available. The first set of data obtained by Lynn Baker and Felipe Soberal (both NAIC) has been reduced, and does suggest that the surface accuracy has deteriorated significantly since it was last adjusted in 1987. This is in part likely a result of settling of anchors under the dish which hold the tie back cables which determine the shape of the cable network, and in part may be a consequence of the upgrade work. Those present during that period know there was a lot of "stuff" dropped on the dish surface, as well as real damage to some of the cables by one major mistake by the contractor. All of this suggest that it should be possible to make a significant improvement in the antenna accuracy in the months ahead. We do need a run of good weather during which we can confirm and complete this first set of photogrammetric measurements. We recently completed the Flag Plaza, a small project on the way to the Visitor Center. It welcomes the visitors to

The new flag plaza seen by visitors en route to the Visitor Center. (Photo by Tony Acevedo)

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our site in the name of Cornell University and the NSF. The number of visitors has remained constant at a level of about 120,000 per year, of which about 30% are children. At the Center we have started the construction of the new Learning Center. In addition, we hope soon to commence work on the planned additional Visitors Quarters which will further enhance our role in science education, enable us to host scientific meetings and workshops such as that planned for June of 2001 on Single-Dish Radio Astronomy (see announcement, p. 21), provide accommodation for summer students and extra accommodation for our users at busy times. We also plan to accomodate members of the Arecibo Users and Scientific Advisory Committee (AUSAC) and NAIC Visiting Committee (VC) entirely on site, once the new Visiting Scientist Quarters (VSQs) units are available. Our cafeteria has been expanded and the VSQs remodeled, including new tile floors on the downstairs VSQs. Many visiting scientists have expressed their appreciation for the X-terminals now available in the "downstairs" VSQ units, while others have been more impressed by the improved television reception available in the units on the hill. All agree that the new gazebo and garden offer a new pleasant environment for lunch and informal meetings. Working space for engineering laboratories, as well as office space for engineering staff, scientific staff, and visitors is presently inadequate. We are preparing plans for a much needed expansion of our facilities in the form of a building joining our current Buildings I and II, and hope at some point to be able to fund this project.

Peeps into the Past: The Angel Ramos Foundation Visitor Center Duncan Lorimer With the completion of the new flag plaza, and the beginning construction of the Learning Center (see previous article), the Angel Ramos Foundation Visitor Center has come a long way since its conception in the late 1980s. Riccardo Giovanelli, the site director at that time, was keen to make more of the natural draw that the telescope has to island residents and tourists. It is easy to forget that, even prior to the Visitor Center, some 40,000 visitors came to the observatory each year. In those days the visitor facility consisted of a 5-minute audio tape description of the Observatory, a small display giving details of the construction of the telescope and key scientific results. In addition, the view of the telescope seen by the average punter back then was somewhat limited and not nearly as impressive as one sees from the Visitor Center platform these days where the whole structure is on view. By 1992 the Visitor Center had gained momentum as a viable project and progress was well underway under the leadership of Daniel Altschuler. The plan was to build a visitor center at the base of one of the towers where there was adequate space for a large number of exhibits, a viewing gallery and an auditorium. In addition to providing visitors with the necessary background to appreciate the research going on at the observatory, a major component in the Visitor Center mission was to encourage young people to follow careers in the physical sciences by helping to improve the science teaching and provide local university students experience in a major research center. As early as 1992, a grant from the Angel Ramos Foundation had been obtained; matching funds needed to be found from private and government sources in Puerto Rico. The planned facility would be a unique resource to the island and, thanks to the hard fund-raising efforts of Daniel Altschuler and Rey Medina (and others),

money started to trickle in. Notable contributions were made from the Municipality of of Arecibo, the Puerto Rican Tourism Company, the Government of Puerto Rico as well as several cooperations and individuals. Funds to develop the exhibits themselves were obtained through a grant from the Informal Science Education Program of the National Science Foundation. Jo Ann Eder and Daniel Altschuler oversaw the content design and fabrication (by Zalisk Martin Associates, Cambridge, MA) of the various exhibits that we see in the Visitor Center today. Major contributions from members of the scientific staff went towards these exhibits. The theme "More than Meets the Eye" presents the idea that we can study the physical world in many different parts of the electromagnetic spectrum, with the focus being on the radio astronomy and atmospheric science carried out at Arecibo. Anyone who has wandered around the many exhibits on display at the Visitor Center today will appreciate the hard work that went into making these displays interesting and fun. Construction of the Visitor Center building began under contract with HÈctor Varela working in close cooperation with JosÈ Maldonado in July 1995. By this time, over $2,000,000 had been raised towards the building costs. Building work continued at a rapid pace throughout 1996 and the structure was completed in 1997. The inauguration ceremony (reported in NAIC/AO Newsletter No. 21) on March 1, 1997 was by all accounts a grand affair celebrating the significant effort put in by various members of the observatory staff. The impact of the completed visitor Center was immediately seen with over 125,000 people visiting in the first year alone. In addition to regular visitors, teacher workshops have proved phenomenally successful with over 100 science teachers attending the summer workshops organized by JosÈ Alonso who joined the staff of the Observatory as Head of the Visitor Center shortly before the inauguration. More specialized science

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workshops have benefited from the excellent facilities in the auditorium of the Visitor Center, too. Three workshops have been held this calendar year alone (see NAIC/AO Newsletter No. 30 for reports) and plans for a major single-dish summer school are underway for 2001 (see poster on p 21). As somebody arriving at the observatory after all this work was completed, the amount of effort that went into the center is not truly appreciated. Only through peeping into the back issues of the newsletter did I gain an insight into what an achievement it is and all involved should be congratulated on a job well done. As an educational facility for astronomy and aeronomy, the Visitor Center is certainly the one of the most informative centers around. The exhibits and presentations are constantly being improved, too. A hyperexhibit where visitors can navigate their way around a collection of astronomy-related images and sites on an html browser is now implemented and proving very popular thanks to the hard work of Tony Acevedo, JosÈ Alonso and others. Finally, recent visitors to the observatory will have no doubt seen the 20-minute behind-thescenes documentary film "A Day in the Life of the Arecibo Observatory" which premiered earlier this year and featured some memorable performances from certain staff members. While none of us have gotten any offers from Hollywood on the strength of this cinematic gem, the movie is certainly worth checking out as is the rest of the Visitor Center next time you visit the Observatory.

the University of Puerto Rico ­ RÌo Piedras. One, Carlos Vargas, continued his work of last summer on hot molecular cores in regions of massive star formation with Peter Hofner. The other, Karin MenÈndez, helped Carmen Pantoja to develop a dipole antenna and receiver which can be used by high school and college classrooms as a hands-on introduction to radio astronomy. Former REU student Anil Seth (1997) returned for a month to continue his summer student project with Chris Salter, reducing multifrequency VLA data of supernova remnants. The NSF Research Experience for Undergraduates program sponsored nine undergraduates and one teacher. One of these students was a member of the UPR ­ Humacao REU program but did her research at Arecibo Observatory. The NASA Capability Enhancement program, based at the University of Puerto Rico, sponsored two; a third was supported by a Mellon Minority grant at Williams College and by the Puerto Rico Community Foundation.

The students had many opportunities to observe with the Arecibo dish. All the students were involved in the ongoing summer student observing project, supervised by Murray Lewis. Their monitoring of the spectra of three lines of OH maser emission from OH/IR stars (evolved stars with circumstellar shells) have revealed many interesting and unexpected results over the three years that the project has been conducted. Comparisons of the post-upgrade spectra with pre-upgrade spectra have revealed several sources with dramatic changes which may indicate the evolution of the mass loss system of the stars. Three other students had their own observing programs as their summer projects. They designed and performed all of the observations themselves, reduced and analyzed the data, and will present the results at the AAS meeting in San Diego. (See below for more details.) The housing situation was much better this past summer. All of the students lived on Observatory property. The boys shared one of the family units on the hill

The 2000 NAIC Arecibo Observatory REU Program Jo Ann Eder Arecibo Observatory hosted fourteen students from colleges and universities in Puerto Rico and in the mainland between May and August. Included in this number were one teacher from a local high school and two students supervised by the Observatory's adjunct staff from

The 2000 REU Summer Students: Standing, from left to right, are Homero Cersosimo, Carlos Vargas (a 1999 Summer Student), Miguel Irizarry, Karin MenÈndez, Alyson Brooks, Shawn Allison, Diane Chin, Amanda Kirschner, and Sarah Boswell. Kneeling, from left to right, are Darik VÈlez, Justin Kinney, Laura Hainline, and Ruth Murray.

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while seven girls shared a house just outside the Observatory gates. NAIC had recently acquired the house with the view of constructing new VSQ's on the site. Crews from the maintenance department did a great job renovating the old house on the site so that the students could live there. There was only one bathroom (for seven girls!) but the shower at the pool was not far away. When the Esperanza community water failed for several days, the crew connected the house to the Observatory water supply so the girls would at least have water. Since the students lived so close to each other, they could share communal meals and socialize. Several staff members generously offered their cars to make sure that the students had rides to the stores, movies and beaches. Organized excursions almost every weekend kept the students from getting "cabin fever". Four of the students took the opportunity of warm clear waters to become certified scuba divers. Besides many informal get togethers, Daniel Altschuler hosted a cookout at the pool for the summer students and their advisers. 2000 SUMMER STUDENT PROJECTS Supported by NSF REU Funds: Shawn M. Allison (Penn State University) worked on several aspects of atmospheric science. His main project, supervised by Qihou Zhou, involved radar observations of meteor echos. Previous Arecibo meteor detections showed a puzzling lack of trail echos ( only 10 out of 367) with the VHF system. Shawn investigated the possibility that this lack could be due to the fixed vertical orientation of the Arecibo dish. He analyzed meteor data from the Mu radar in Japan which is stearable. Many meteors were detected, most displaying head echoes but several with trail echoes. Shawn also assisted Jonathan Friedman with lidar observations. Sarah Boswell (Bethel College) worked with the aeronomy group (par-

ticularly with NÈstor Aponte and Sixto GonzÀlez) on two web design projects: the Arecibo Incoherent Scatter Radar Database (AISRD), and a web page that provides up-to-date lists of publications by various members of the aeronomy group. The AISRD provides access to the Arecibo World Day data through a convenient graphing interface. On an image request form, the user selects from appropriate options for the ionospheric altitude region and date of interest. A fully-interactive image provides height and time profiles with adjustable colorbar scales. Sarah hopes that her web page will be a valuable tool for aeronomers around the world. Alyson Brooks (Macalester College) mapped portions of the Magellanic Stream in HI with the Arecibo Telescope. She and Snezana Stanimirovic developed their own on-the-fly observing scheme of driving in RA and then stepping in Dec for individual 10â11 small maps. These maps were then combined to make a map of the regions MS V and MS VI. With a velocity resolution of 1.3 km s ­1, the kinematics of the regions could be studied, as well as the structure. Several interesting features were revealed. In MS VI, the tip of the Stream, three clumps were seen, one of which had some head-tail structure and another which was extremely dense with a velocity gradient. MS V had no discernible clumps but appeared to have two distinct sheets of gas, separated by about 30 km s ­1. The maps are being analyzed further to test various theories of the origin of the Magellanic Stream and to determine the hierarchy of structure present. The results will be presented in a poster paper at the winter AAS meeting. See also page 2 of this issue for further details. Diane Chin (SUNY­Binghamton) searched for extended HI envelopes around low surface brightness (LSB) galaxies in collaboration with Karen O'Neil. The existence (or lack of it) of an HI envelope might shed some light on the evolutionary history of the LSB

galaxies and explain their extremely low background star formation rates. Extended envelopes could also have important implications in the search for the source of Lyman- absorption at low redshifts. Diane observed the regions surrounding 4 previously detected LSB galaxies. Three of the four galaxies showed signs of extended HI, although two of them had suspiciously asymmetric distributions. While no detections were made around the remaining galaxy, the upper limits on its HI content were too high to rule out the existence of an extended envelope. Diane's observations will be presented in a poster paper at the winter AAS meeting. Ernesto Estremera (Quebradillas High School) was the 2000 Teacher-inResidence at the Angel Ramos Foundation Visitor Center, under the supervision of young JosÈ Alonso. Ernesto shared his longtime hobby of studying local plants and wildlife by developing an exhibit for the Visitor Center. He obtained wonderful digital images of the flora and fauna of the Observatory grounds and categorized all the varieties he found. The images will be part of a new hypertext exhibit to expand the range of topics to biological subjects. His final presentation impressed all of the Observatory staff with the wealth of beauty in our everyday environment. Laura J. Hainline (Indiana University) studied the kinematics of dwarf irregular (dIrr) galaxies with Jo Ann Eder. Laura observed high resolution (0.65 km/s) HI spectra of a sample of dIrr galaxies for which previous low resolution (8 km s­1) detections indicated narrow single-peaked profiles. A single-peaked profile could result from a face-on orientation or from a small pressure supported galaxy with little or no rotation. However, the high resolution spectra (0.65 km s­1) of 9 of the 15 galaxies displayed the double-horned profiles indicative of a rotating gas disk. The velocity dispersion for these galaxies was estimated from the wings of the profile and generally compared well with the esti-

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mate obtained from the remaining single-peaked profile of a face-on dwarf. Estimates of the dynamical masses for the double-horned sample indicate high mass-to-light ratios, as compared with other disk galaxies. Laura has already prepared her poster for the winter AAS meeting in San Diego. Justin B. Kinney (Cornell) applied his love of mathematics to a very sticky problem -- understanding the origin of the large wavenumber tail shown by the spectra of atmospheric gravity waves. Lily Childress, a 1999 AO REU student working with Mike Sulzer, found that Colin Hines' Doppler spreading theory produced the tail, but with a k­1 dependence rather that k­3 as found experimentally. Justin tried many approaches to reconcile the theory with experiment -- one-dimensional approximations using Gaussian impulses (exponential tail), using a cosine wave (k­1 tail), and a twodimensional special case of two gravity waves intersecting perpendicularly (k­1 tail assuming a unity Jacobian and k­3 tail with the exact form of the Jacobian). Justin concludes: "This is a hard problem but it is still accessible. I intend to keep working on it." Amanda Kirschner (Carleton College) studied pulsar dynamic and secondary spectra for data obtained by her adviser, Ramesh Bhat, with the Ooty Radio telescope in India. Dynamic spectra (intensity vs. time and frequency) is the result of scintillation of the pulsar signal by the interstellar medium (ISM). The 2D Fourier transform of the dynamic spectrum produces a distribution of points in the plane of inverse frequency and inverse time, called the secondary spectrum. The secondary spectrum can reveal features such as "islands" (ie, periodicities in the dynamic spectrum signifying fringing events) or "wisps". Wisps appear as faint diffuse features extending outward from the origin of secondary spectra. First recognition of wisps came from recent Arecibo observations (see NAIC/AO Newsletter No. 29), and their study is a topic of contemporary research. They are found to lie

significantly above the noise floor. The exact cause of wisps is still unknown; they may be either intrinsic to the pulsar or caused by the ISM. Amanda analyzed data for a dozen pulsars. She found no new islands but did find at least four pulsars with evidence for wisps. Amanda described some of the basic properties of these wisps which may help to explain their nature. These results will be presented in a poster paper at the winter AAS meeting. Ruth Murray (Harvard) carried out two investigations of pulsar positions with Duncan Lorimer. For the first, Ruth evaluated the unexpected observed bias in the offsets between pulsars and their associated supernova remnants. While the offsets were evenly distributed in galactic latitude, they showed a marked preference for one direction in galactic longitude. However, Ruth's analysis showed that the errors in the supernova remnant center positions are significantly larger than previously estimated, making the offset result more marginal. The second investigation involved creating a Monte Carlo simulation of the observed pulsar population in an attempt to differentiate between different possible radial distributions for galactic pulsars. The simulation was then applied to the investigation of pulsar/supernova offsets. Supported by Other Funds Darik O. VÈlez (Williams College), under the guidance of Sixto GonzÀlez, processed over 400 hours of radar topside ionospheric data and calibrated it with Arecibo ionosonde data. This observational data was then compared with the predictions of the International Reference Ionosphere (IRI). Darik found that the IRI model does not predict the hydrogen and helium ion fractions and densities for the topside level of the atmosphere. Contrary to the model, the data indicates that the hydrogen ion percentage continues to rise with height while the electron density continues to fall. The data also shows a variation in the height of the peak hydrogen density

from one night to another. Other discrepancies appeared regarding the intensities and height of the peak helium ion densities. Clearly, the IRI needs to be reevaluated with respect to the topside layer. Homero Cersosimo (Univ. of Puerto Rico ­ Humacao), an applied physics major and the son of a former AO postdoctoral fellow, spent the summer using new engineering computer packages on various projects, under the supervision of Edgar Castro. Homero's favorite project was building and testing a YAGI antenna. He will use the antenna for his ham radio setup at home. Homero also designed a cabinet for the upgrade of Carriage House 1. The upgrade will replace the existing equipment currently housed in three bulky cabinets with more compact versions in one cabinet. Because of support structures and existing doors, the design of this cabinet was very tricky but Homero found a configuration which would work. Homero also mapped all of the AO Fiber Optics, which were recently installed, on a document which can be used for maintenance and repairs -- a critical contribution to the future of the Observatory. Miguel F. Irizarry (Univ. of Puerto Rico ­ Mayaguez), an engineering student, also worked with Edgar Castro in the Engineering department on the Carriage House 1 upgrade. He investigated the modern equipment to be put into Homero's cabinet which could be used to replace the older, more bulky, equipment. He also designed a cold load which could be used to calibrate the system temperature of the receivers in the Gregorian dome. Both Homero and Miguel found the practical experience they received at Arecibo Observatory to be a good complement to their course work. Karin MenÈndez (McGill University) was a member of the REU program at UPR ­ Humacao where she spent several weeks completing computer workshops. However, she worked on a very interesting educational project at AO with Carmen Pantoja. Karin and Car-

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men designed and built a dipole antenna and receiver in order to observe the radio emission from Jupiter. The antenna, located at the helipad, worked well but Jupiter never rose far enough above the horizon to be detected. Carmen will try later in the year when Jupiter is more cooperative. However, Karin and Carmen demonstrated that the antenna is a low cost and instructive means of introducing radio astronomy in the classroom at either the high school or undergraduate level. REU 2001 Applications for the year 2001 REU program at the Arecibo Observatory are available from Jill Morrison, NAIC, 504 Space Sciences, Cornell University, Ithaca, N.Y. 14853. Her e-mail address is morrison@astrosun.tn.cornell.edu.

minimum backend requirement is to replicate the wideband Arecibo pulsar processor (or equivalent) for each pixel so that at least 100-MHz bandwidth per pixel available for spectral line and pulsar work. With this in place, a number of large-scale projects (currently unreasonable in terms of telescope time required) then become feasible. At the same time, there will certainly be smaller proposals for selected area searches and mapping that will be judged meritorious and will benefit appreciably from the availability of the array. We are currently working to define better the characteristics of such a focal plane array. Our baseline thinking is a 7-element (central element plus hexagonal ring) array covering 1100 to 1500 MHz. This follows in many ways the very successful 13-element array implemented on the Parkes telescope. Two things that make this plausible from a financial viewpoint is that the backends will essentially be the same for spectroscopy and pulsar research, and that the cost of fiber optic transmission systems for the IF have come down significantly in price. Much detailed design needs to be done, and this is already starting. Some salient questions include physical rotation of the array for parallactic angle tracking, and the possibility of future generations of backend systems to include greater bandwidths. If readers are interested in the array from either scientific or technical viewpoints, and have suggestions or wish to become more involved in the design phase, please contact Paul Goldsmith through email (pfg@astro.cornell.edu).

the Physical Sciences at Cornell University will be representing NAIC in the national and international arenas. At the Observatory, Rey VÈlez has taken up the position of Arecibo Observatory Spectrum Manager. He brings to this position his long experience as Head of Telescope Operations (a job now taken over by JosÈ Cruz) as well as his expertise in amateur radio. Rey will be coordinating the efforts of the SMARG. Tapasi Ghosh will continue to serve as the scientific advisor to the SMARG after five years as Local Spectrum Manager. Edgar Castro and Phil Perillat will provide hardware and software support. Chris Salter, Mike Sulzer and Daniel Altschuler will continue as members of the SMARG. The group meets once a month and can be reached using the mailing alias: rfi-grp@naic.edu.

Development of a Focal Plane Array System for the Arecibo Gregorian Paul F. Goldsmith The idea of using an array of feeds with the Gregorian is a very plausible one, although the relatively fast, off-axis system configuration means that the aberrations need to be examined carefully. A study was made even before construction was started (Kildal et al. 1993, IEEE Trans. Ant. Propag., 41, 1019), which found that a 7-element array for L-band was feasible. While we have considered more advanced concepts, such as an array that samples the electric field distribution in the focal plane rather than the power distribution, we have come to the conclusion that if we want to get an array on the telescope within the time frame of a few years, we have to adopt a relatively straightforward design. There is ample scientific justification for such an array. Jim Cordes and Dunc Lorimer, assisted by Chris Salter and other NAIC scientific staff members, have been putting together a scientific "white paper" that includes discussions of pulsar searches and 21-cm mapping for Galactic and extragalactic studies. A

Recent NAIC colloquium talks Karen O'Neil and Duncan Lorimer The 5 colloquium talks given since the last Newsletter appeared (August, 2000) are listed below. Information on these and all upcoming talks at the observatory can be found at: http://www.naic.edu/~astro/talks 12 September, Prof. J.V. Narlikar (InterUniversity Centre for Astronomy and Astrophysics), The Big Bang: Theories & Alternatives 13 September, Prof. J.V. Narlikar, The Strange Effects of Gravity (Public Talk) 12 October, Snezana Stanimirovic, (NAIC), The interstellar medium in the Small Magellanic Cloud 20 October, Matthew Wyndham, The political elections in Yugoslavia

Reorganization of RFI efforts Daniel R. Altschuler With the leave of absence of Mike Davis and the departure of Zach Barnes, the Spectrum Management and RFI-protection Group (SMARG) has been reorganized in order to ensure smooth functioning in these areas. Yervant Terzian, the David C. Duncan Professor in

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Computer Department News Arun Venkataraman One year into the Observatory's "T1" link to the Internet, although it is a vast improvement over the "connection" we had before, we find (surprise!) that peruser bandwidth is frequently insufficient. Enter Internet 2, which promises research users (only!) a "T3" (45Mbits/s or 30âT1s) speed connection to other Internet 2 sites. Following the funding early this year of the joint proposal to NSF/CISE by the University of Puerto Rico and the Observatory for Internet 2 connectivity and receipt of bids from vendors this month, construction of the new links is slated to begin in late 2000 with connectivity in early 2001. The new "Americas 2" undersea fiber will be the prime pathway for the off-island Internet 2 connection, which will link the on-island GigaPOP (located at the UPR High Performance Computing Facility in Rio Piedras) to the Abilene backbone at the Florida GigaPOP. Applications for the increased bandwidth include expanded remote observing facilities via a "virtual Control Room", distributed computing between cooperating centers for quick turnaround of observational results, and collaborative projects using multiple instruments to provide realtime views. "Regular" Internet access (e.g dotcoms) will be discriminated out by routers at each Internet 2 institution via the Border Gateway Protocol, thus freeing the wires for serious research. Continuing the modernization of the Observatory Local Area Network, new cable paths and switched 100 Mbit fibers (as well as telephone wire) have been installed in the Operations Building. Switched 10/100 ports are being installed in the Receiver Room, Control Room and Equipment Rooms, as well as dedicated, UPS-regulated (and colorcoded!) power lines. Switching capacity has been installed for additional 10/ 100 ports in the Operations Building and VSQs (present and future). All the desktop Sun Ultra-5s and 10s have received memory upgrades for
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minimum of 256 MBytes; most Ultra10s have also been upgraded with a second internal 20GB disk (this also requires Solaris 7 or 8). The central server was upgraded to a dual-450MHz UltraSPARC-based Sun Enterprise 420 with 2GB memory. Two high-speed Exabyte Mammoth-2 tape drives (12 MBytes s­1, 60GB) have been acquired for data acquisition and offline dumps to try to keep up with disk performance (25 MBytes s ­1) and capacities (72 GBytes). Linux continues to make inroads on the AO computing scene, with further development of the WAPP (Wideband Arecibo Pulsar Processor) using a dual-processor system, and an increasing number of desktop PCs. Workhorse black-and-white printers are network attached HP8000DN duplexing units, replacing former printers attached to Sun servers. A networked color inkjet (Lexmark OptraColor 45N) is also available. Control Room News JosÈ Cruz The AO-CONTROL, graphical user interface (GUI) is the latest thing happening in the control room. The GUI has been in use remotely, and by our staff and visiting observers at an increasing rate. Remote observers' and visiting observers' feed back of problems and suggestions to the GUI are welcome and very useful. The operator is still the main link to the telescope for all observers. The control room has gone through some changes but still looks like the old control room we love so much. A project to install new cabling of the control room and receiver room (fiber optics upgrade) for data-acquisition, links, phones and regulated power connections is in progress. Following the transfer of Rey Velez to the RFI-Protection Group (see page 19) and the reduction of one operator, our telescope operations staff that once is now down to 6. We hope to hire a

service observing specialist in the near future to help keep the control room adequately staffed. We are, as always, ready and willing to assist and help the visiting observers and staff in any way shape or form possible.

Comings and Goings Bryan MacPherson Sixto GonzÀlez Bryan MacPherson left the observatory during the summer to take a job in private industry. Bryan came to us in 1997 from the University of Sheffield in England where he had used Arecibo topside data as part of his Ph.D. thesis project. Bryan spent three years at Arecibo and was responsible for the theoretical modeling component of our topside program in the Space and Atmospheric Sciences department. In addition to his modeling expertise he was a great asset to the after-hours social life at the observatory, and was the life and soul of many a party down by the pool and at the beach. The atmosphere was lively and full of jolly banter whenever he was around and we are sorry to see him go. We wish Bryan great success in his new career working with UPS in Alberta, Georgia starting in early 2001. Jeff Hagen Arun Venkataraman We are pleased to welcome Jeff Hagen once again to the Computer Department, this time as an NAIC employee. Readers of these pages will remember Jeff 's sterling contributions to the AO during his leave of absence in 1999 from NRAO/Tucson. In particular the Wide-

Jeff Hagen

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band Arecibo Pulsar Processor (WAPP) conceived and designed by Bill Sisk, Andy Dowd (also ex-NRAO, now with The Math Works) and Jeff has since evolved into a versatile backend processor, leveraging the strengths of off-theshelf PC hardware and the open-source Linux operating system. Jeff is currently continuing development of the WAPP (to be upgraded soon to use newer faster CPUs) and reusing it in an aeronomy radar data acquisition application. We extend a special welcome back also to Jeff 's wife Angela who, in addition to livening up the local music scene, has taken up English and journalism teaching at an Hatillo high school.

Andy Dowd Eddie Castro Andy worked at the Observatory between June 1998 and May 2000. He was hired as a Motion and Control Engineer to work with our sophisticated drive systems. During this short stay he redesigned some parts of the drive system, making them very reliable and precise He also played key roles in many projects such as the WAPP and VLBA. Without Andy's skill and dedication, many of these projects would not have come to fruition. Andy was a pleasure to work with and, along with his wife Zoe, greatly enriched the social scene of the observatory during his two-year stay. Andy is now employed at the MathWorks organization and he and Zoe are now living in Boston. We are very sorry to see them leave and wish both of them our best. Zach Barnes Eddie Castro Zach was hired as an RFI Engineer. He worked at the Observatory between the summer of 1999 and the summer of 2000. During this short stay he developed some software to search for interference. He also built the hardware for the mobile RFI search vehicle which is now in regular use by Rey VÈlez. Our best to Zach.

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NAIC/AO Newsletter is published three times per year by the National Astronomy and Ionosphere Center. The NAIC is operated by Cornell University under a cooperative agreement with the National Science Foundation. Duncan Lorimer and Jonathan Friedman, Editors Address: NAIC/AO Newsletter HC03 Box 53995 Arecibo, PR 00612 Phone: +1-787-878-2612 Fax: +1-787-878-1861 E-mail: dunc@naic.edu or jonathan@naic.edu WWW: http://www.naic.edu

NAIC/Arecibo Observatory Newsletter 504 Space Science Building Cornell University Ithaca, NY 14853-6801 U.S.A.

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